An upstart challenger is adding some muscle to its claim that it is the legitimate leader of the Taliban.

In the week following its announcement that it was splitting from the Taliban, the High Council of the Islamic Emirate of Afghanistan has borrowed from the militant group's formal name, chosen a leader, and rallied support in Afghanistan's west.

Now it is directly taking on Taliban fighters in Zabul Province, in the south of the country, in what are seen as the first major fighting between rival Taliban factions.

Mohmand Nostrayar, head of Zabul Province's Arghandab district, has said that at least 50 militants have been killed and dozens injured in heavy clashes since late November 6.

The splinter group says it does not acknowledge Mullah Akhtar Mansur, who assumed power after Kabul announced in July that Taliban founder Mullah Mohammad Omar had been dead for two years, as the rightful leader of the Taliban.

Instead, the group has named Mullah Mohammad Rasul as its leader, putting it in direct conflict with the widely accepted Mullah Mansur and his supporters.

Mullah Mansur's fighters have clashed in Zabul with fighters loyal to Mullah Mansur Dadullah, the deputy head of the splinter group, and who are reportedly being aided by fighters from the Islamic Movement of Uzbekistan (IMU).

Commenting on the fighting in Zabul, Arghandab District head Nostrayar said that the Taliban splinter group had been joined by IS militants in southern Afghanistan.

And a Taliban commander loyal to Mullah Mansur told AP on November 8 that Mullah Rasul's faction had joined forces with IS gunmen because it did not have enough fighters.

The IS group, which controls large areas of Iraq and Syria, has been slowly building a presence in Afghanistan. Militants fighting under the IS banner hold small pockets in the eastern Nangarhar Province, bordering Pakistan, and established a presence in Zabul earlier this year.

Mullah Rasul told a rally in Afghanistan's western province of Herat on November 7 that IS fighters were "brothers," but that his faction would not let them into Afghanistan. The rally took place just several kilometers from Shindad Air Base, a major base used by Afghan and NATO forces.

Mullah Rasul was named as the leader of the splinter group in a mass gathering of dissident fighters on November 3, and Mullah Dadullah was named as his deputy during a meeting held in the remote southwestern province of Farah on November 1.

Abdul Manan Niazi, spokesman for the splinter group, told RFE/RL's Radio Free Afghanistan on November 7 that the group's mission was to "unite the Taliban" and urged his followers not to start an internal Taliban war.

Niazi, the former governor of Herat during the Taliban regime, accused Mullah Mansur of taking the leadership through a "coup" and added that he was not the "legitimate leader."

Niazi added that the group would hold gatherings in a number of northern provinces, including Badghis and Faryab, in the coming days to garner support for the new faction.

Mullah Rasul, who was believed to be close to Taliban founder Mullah Omar, served as the governor of southwestern Nimroz Province during the group's rule.

Mullah Mansur has been struggling to quell the internal dissent within his group and reconcile feuding factions since the bitter leadership transition.

Some Taliban commanders have accused Mullah Mansur of covering up Mullah Omar's death and assuming the leadership without a vote.

It is unclear whether the new splinter group will rally wide support but its emergence poses a fresh hurdle to potential peace talks with the government.

Written by Frud Bezhan, based on reporting by Radio Free Afghanistan correspondent Shahpoor Saber in Herat.